Definition of trebly in English:

adjective (treblier, trebliest)

Starting out with breakneck speed and the chaos it inspired, they played hard and loud with an abrasive, trebly sound that went straight for your jaw muscles.

The mono soundtrack sounds predictably canned and trebly, but the dubbed dialogue (there's no audio option for the original Spanish) presents clearly and the score, such as it is, is more or less unobtrusive.